This document describes a proposal to investigate the relationship between hormone status and development of male breast cancer. Two approaches will be used. The first will be a case-control interview study of 73 "other white" patients under age 75 at diagnosis and 73 such age-matched neighborhood controls. All patients will be obtained from the files of the Los Angeles County/USC Cancer Surveillance Program, from which about 22 such cases under age 75 are identified annually. Data will be obtained by personal interviews conducted by a trained nurse epidemiologist. Questionnaire items will focus on factors known to effect hormone levels including obesity, liver disease and exposure to certain drugs. Data will also be collected on marriage and reproductive history, prior breast disease, family history of breast cancer, diet and other possibly relevant conditions such as radiation. The second approach will be to compare endogenous serum hormone levels in patients, their brothers and controls. Specimens will only be obtained from patients treated by surgery alone and who, at time of specimen collection, have no active disease or other illness. Hormones to be measured include estrone, estradol, testosterone, and prolactin. Sex hormone binding globulin will also be measured. For comparison with previous studies, 12 hour urine samples will be collected and assayed for conjugated estrogen fractions. Buccal smears will be obtained on all individuals and tested for sex-chromatin.